Medicinal Plants as Sources for Drugs and Vaccines

Abstract

In general, vaccines are important biological factors that stimulate human immunity to resist various diseases or their pathogens that invade him. The vaccine includes protein material of the pathogen itself, which is either killed or weakened form, or is made from corresponding artificial protein subunits to help human’s immune system for recognizing antigens. However, it has been observed that there are some side effects appeared from using of traditional vaccines, which made trending toward finding alternative solutions is an important goal. In recent years, with the progress in medicinal sciences, genetics and plant biotechnology, the concept of edible vaccines has emerged by biotechnologists in an attempt to use edible plants in the production of alternative vaccines for commercial vaccines that are useful in treating diseases that affect humans without needing for injection or refrigerated storage, which is done through genetically engineering plants to carry antigens through several methods, like bacterial vectors, shot gun or microinjection through plant tissue culture techniques to produce vaccine-bearing plants like banana, maize, potato, rice, tobacco, tomato, legumes and others which makes these plants have two tasks, their suitability for food and to stimulate the body’s immune response against many pathogens at once.

1. Introduction

Vaccination is a simple, safe, and effective way to protect people from harmful diseases before they are exposed to them via induction of the body’s natural defenses to build resistance to specific diseases, as well as strengthen the immune system. Vaccines train the immune system to make antibodies (proteins that the immune system naturally produces to fight disease), just as it does when exposed to a disease, as it recognizes an invading pathogenic organism, such as a virus, bacteria, or others, and fights it. However, because vaccines contain only dead or weakened forms of germs such as viruses or bacteria, they do not cause disease and do not expose the human body to the risk of complications, so most vaccines are given by injection, while others are given orally or sprayed into the nose to treat many diseases such as hepatitis B, measles, tuberculosis, tetanus, diarrhea, diphtheria, etc. [1].

Our immune systems have the ability to remember. Once exposed to one or several doses of a vaccine, we usually remain protected from the disease for years, decades, or even for life. This makes vaccines so effective, as they aim first to protect us from the disease before resorting to treatment after infection [2]. Although traditional vaccination is the safe method used around the world to confront the risk of diseases when exposed to them, it faces some limitations, represented by the cost of production, storage, distribution and the lack of sufficient scientific research on it. Therefore, scientists and researchers have turned to finding safe therapeutic alternatives under the progress made in various biotechnologies and genetic engineering, which made it possible to produce genetically modified plants, which prompted researchers to introduce anti-pathogenic genes into these plants in order to produce plants that are eaten and carry the anti-gene at the same time, which in turn are easy to transport, distribute and store so that they are available to humans when administered as edible vaccines [1, 3].

The concept of edible vaccines, plant-based edible vaccines, or plant-based vaccination appeared in the twentieth century. This is called a GreenVax (a concept developed in the 1990s, which means the consumption of edible tissues of transgenic plants), which refers to food, typically plants, that produce proteins, vitamins, or other nourishments that act as a vaccine against a certain disease. Once the plant, fruit, or plant-derived product is ingested orally, it stimulates the immune system. Specifically, it stimulates both the mucosal and humoral immune systems [4, 5, 6]. Edible vaccines offer many benefits over traditional vaccines due to their lower manufacturing cost and lack of negative side effects. However, there are limitations as edible vaccines are still new and developing. Further research will need to be done before they are ready for widespread human consumption.

The plant-based vaccine method works by isolating a specific antigen protein, which triggers a human immune response from the target virus. A protein gene is transferred to the bacteria, which is then used to “infect” plant cells. The plants then begin producing the exact protein that will be used for the vaccine [7]. The possibility of introducing a set of genes of human pathogens (whether viruses or bacteria) into plant cells, thus re-cultivating the plant again so that it can produce biological primary vaccines containing pathogen genes, and by feeding the tissues of these plants to humans or animals, an immune response to vaccines is elicited. The new process will only take 4–6 weeks. Depending on this, if the project succeeds, it will be one of the largest and most powerful vaccine facilities in the world. However, the development and widespread use of new vaccines to improve health conditions at the global level face many challenges. The cost of the new vaccine must be low, the vaccine must be administered orally without injection, and it must remain stable in high temperatures. It should also contain a combination of vaccines to prevent diseases prevalent in developing countries [8, 9].

2. Why the plant-based edible drugs and vaccines are important?

Different drugs and vaccines were used in all countries of the world, as they caused a clear decrease in the death rates among humans, which are caused by various microbial infections with a large percentage, whereas in some cases vaccination leads to death of the vaccinated person [8]. The use of plants that are eaten to act as edible vaccines is an effective, safe alternative to traditional vaccines in controlling various types of diseases and illnesses [3, 10]. To obtain an edible vaccine, the required gene that encodes that active compound as a vaccine is selected and inserted into the desired plant, where this plant manufactures the proteins encoded for this vaccine to perform a systemic immune function that gives the required immunity to the body of the organism when the plant is eaten [11, 12].

Regardless of the way that edible vaccines are consumed, they all share an important common goal of immunizing the human body against different pathogens or before they multiply in quantities that are sufficient to cause disease and the appearance of disease symptoms in the patient. It is well known that the traditional methods of immunization against diseases are done by exposing the person’s immune system to killed or very weakened bacteria or viruses [13]. Therefore, when the immune system becomes sensitive to any foreign organism in the vaccine, it will act as if the body is under attack and mobilize all of its forces to eradicate and destroy the attacker after targeting the antibody gene, which the immune system distinguishes as foreign proteins that have entered the body. In fact, there is a rapid suppression of the response, but it leaves behind a guard or a watchdog in the memory of the cells that remain fully prepared when the pathogen enters the body in the future, so some vaccines and serums provide the body with lifelong protection, and the other section fades after time, such as the cholera vaccine and the tetanus vaccine, which requires periodic immunization. It is noted that traditional vaccines have few risks, the most important of which is that the organisms with which the body was vaccinated may live and multiply inside the body, causing diseases that were supposed to be eliminated. From this aspect, most vaccine manufacturers today prefer another type of vaccine called subunit preparations, which consist mainly of antigenic proteins that have been discovered from the genes of the pathogens. Thus, there will be no possible chance for infection to occur in the future. Despite the importance of this modern industry of subunit vaccines, they are criticized for their high production costs due to their manufacture from bacterial cultures or animal cells, as well as their high purity and need for freezing.

The edible vaccines, which are the focus of our discussion, are similar to the subunit preparation in terms of being genetically engineered to ensure that they contain the antigen and do not contain the organism that causes the disease, and both are safe to use. Before starting the production of edible vaccines, scientists raised a number of questions, including whether plants that will be genetically modified to contain the antigen able to produce effective copies of the intended protein. Will the antigen transferred to edible plants be destroyed when consumed by humans or animals? Will the gene decompose in the stomach before it performs its role compared to subunit preparations that are given as injections to avoid their damage? Will the antigen that was produced alert the human or animal immune system, and will the immune system’s response be sufficient to the extent of protecting humans or animals from infection with the disease against which they have been vaccinated? Besides that, researchers must know whether the edible vaccine appears in the mucosal immune system because many pathogens enter the body via the mouth, nose, reproductive organs, and others [14, 15, 16].

It is noted that when the response of the mucosal immune system is effective, molecules known as secretory antibodies are generated, which are released into the vacuoles of the orifices to resist the attack of pathogenic organisms that they find. An effective interaction may occur that activates the immune system in the body cells and thus kills the attacking pathogen. As it is known, vaccines injected into muscles avoid the mucous membranes, so the immune response to these membranes is weak, while edible vaccines come into contact with the internal walls of the digestive system, so it is assumed that they activate both the response of the mucous membranes and the systemic immunity in the body. It is assumed that this dual effect provides protection against dangerous microorganisms, especially those that cause diarrheal diseases, which prompted researchers to prioritize their research in combating diarrhea causes first, then other pathogens, especially Norwalk Rotavirus [17, 18], against the Escherichia coli bacteria [19] that secrete internal toxins causing what is known as traveler’s diarrhea, which leads to the death of nearly three million children annually in third world countries, and Vibrio cholerae (the bacteria that cause cholera) [20].

In fact, ideas began to circulate among many researchers from different countries of the world since 1995. For example, a gene encoding a protein was isolated from the virus that causes hepatitis B virus (causing liver damage and liver cancer) and transferred to the tobacco plant, which stimulated this plant to protein manufacturing. After injecting the antigen into the mice, it led to the activation of the components of the immune system of the mice to the same extent as what happens when they are infected with the hepatitis virus [21].

3. Methods of preparing the edible vaccine

Several methods exist for genetically modifying plants to obtain edible vaccines, such as gen gun, vector system (bacteria), chimeric viruses, and electroporation (Figure 1).

Figure 1.

Schematic representation of various methods for developing an edible plant vaccine.

The most important method adopted in the production of an edible vaccine is the vector carrier method, which depends on the Agrobacterium tumefaciens bacteria as an intermediate vector in the transfer of the genetic material (antigen proteins) from a virus or other bacteria to the target plant, which is embodied in the immune response of the organism after consuming calculated amounts of the antigen-bearing plant. This process is performed using plant tissue culture technology [22, 23]. It is also possible to insert the desired DNA into the plant genome by direct methods in plant genetic engineering. Perhaps it is easier to use the bombardment method by means of an gene gun after shooting it in the cultures of plant embryonic cell suspensions, because they are specialized into embryos, and it is easy to grow into a complete plant carrying the desired gene.

Regardless of how the plant is modified, the desired DNA randomly pairs with the plant’s genome, producing different levels of antigen expression that differ from one plant to another. Thus, it is preferable to modify 50–100 plants at the same time, and each plant is considered a separate line, and through this number, the line (plant) that is more expressive of the antigen with less negative effects on the body is elected. The method of producing the edible vaccine using A. tumefaciens as an intermediate vector can be summarized in the following steps:

1. Vegetable leaves (leaves from the potato plant, for example, as shown in (Figure 2) are separated in good health condition, sterilized superficially, and cut into explants. When the cutting areas of the explant’s edges increase, there is a greater chance of infecting them with bacteria carrying the required gene, and thus, the success of the genetic transformation process.

2. Exposing the explants to bacteria (A. tumefaciens) carrying the antigen gene and the antibiotic resistance gene in a suitable culture medium which allows the bacteria carrying the two genes to deliver them to the genetic material (DNA) of the plant cell.

3. Exposing the plant cells to the antibiotic to kill the cells that do not carry the new genes and transferring the plant cells that contain the new genes to a suitable nutrient medium to stimulate the formation of callus in an appropriate size.

4. The induced callus mass is transferred to the regeneration medium to form shoots and roots, and the formed plantlets are separated.

5. The plants are acclimatized and transferred to the soil. After 3 months, plants bearing the antigen vaccine are produced. Its gene expression appears in potato tubers, which can be consumed as an edible vaccine.

Figure 2.

Schematic showing the method of potatoes-edible vaccine production using Agrobacterium tumefaciens as intermediate vector through plant tissue culture technique.

Some of transgenic plants were invested to be consumed as edible vaccines, as shown in Table 1.

Crop	Disease to be treated	Gene expression	Product	Reference
Viral vectors in tobacco	Non-Hodgkin’s lymphoma	Antibody	Parts of antibody in single and various chains	[24]
Genetically modified tobacco	Tooth decay	Antibody	CaroRx	[25]
Genetically modified yellow corn and potato tubers	Diarrhea	Vaccine	Thermally stable toxins of E. coli	[2, 5]
Genetically modified yellow corn	Cystic fibrosis, pancreatitis	Therapeutic enzyme	Gastric lipase	[16]
Genetically modified potatoes and lettuce	Viral hepatitis type B	Vaccine	Hepatitis B virus surface antigen	[26]
Transgenic Arabidopsis	Vitamin B12 deficiency	Food	Human intrinsic factor	[27]
Genetically modified yellow corn	Intestinal inflammations	Food	Lactoferrin	[28]
Genetically modified potato	Norwalk virus	Vaccine	Norwalk virus capsid protein	[18]
Transgenic Arabidopsis	Diabetes	Hormone	Insulin	[29]
Transgenic rice	Diarrhea	Food	Lactoferrin, lysozyme, human serum albumin	[16]

Table 1.

Some pharmaceutical substances that were derived from plants to treat some human diseases.

With the progress made in medical, agricultural, and pharmaceutical sciences, companies from different countries of the world produced vaccines from potatoes, tomatoes, lettuce, spinach, white clover, and Arabidopsis, where these plants were used as hosts for the production of vaccines [30, 31]. There has also been progress in the use of plant species that are not edible plants but are medicinal plants and easy to handle in the laboratories for this purpose, such as Aloe vera plant [32], Neem plant [33], and Chlamydomonas reinhardtii green algae [34]. In general, each plant has its advantages and disadvantages if it is used as an edible vaccine.

4. Advantages of edible plant vaccines

The most important advantages of plants that make them candidates for the production of edible vaccines:

1. Must they have a long shelf life, as large-scale storage methods such as refrigerated storage are not required, which makes it possible to preserve the plant or the edible part of it without spoiling for a long time.

2. They are characterized by their rapid grow ability for producing them in large quantities, and they are cheap for the purpose of purchasing them by the consumer, because some fruits that grow on trees take a longer time to grow, which makes them expensive accordingly.

3. Easy for genetic transformation of most crops that grow as native or local crops in their regions, which facilitates the possibility of producing edible vaccines and the ease of transportation and distribution to the consumer.

4. The edible vaccines can be taken by eating the plant or part of it without the urgent need to purify or process it.

5. Stimulating the immune response on the mucous surfaces lining the mouth (mucosal immunity), which is the first line of defense in the body.

5. Disadvantages of edible plant vaccines

1. Will the antigen be able to survive the acidic conditions of the host stomach? and if it succeeds in that, will it be able to stimulate the immune system in the right way? Although initial trials have shown promising results in humans, it is not clear what will happen when a person who has eaten an edible vaccine comes into contact with the actual virus or pathogen.

2. The most difficult task remains in how to adjust the dose of the edible vaccine, as there may be a risk that a dose that is too high can evoke oral tolerance to invading bacteria or viruses rather than an immune response against them. In addition, the dosage requirements for children and adults are different, so research is continuing to find solutions to these problems.

3. The availability of limited knowledge regarding plant biotechnology leads to negative public opinion and strict regulations, thus discouraging future investment in the pharmaceutical business to produce edible vaccines.

4. Plants are living organisms that change in terms of growth and response to different environmental conditions, so there may not be a guaranteed continuity here for the production of a required edible vaccine.

6. Most important plant species used as edible vaccines

As a result of the successes achieved from the expression of genes introduced into plants or their parts that are eaten, many plants that have been genetically modified have been produced and tested. Table 2 shows the most important of these plants, their merits and demerits.

Plant species	Merits	Demerits	References
Tobacco (Nicotiana tabacum)	A good model for the evaluation of recombinant proteins. The plant can be preserved at a low cost, as it gives large quantities of seeds with the possibility of storing them for a long time. Ease of purification of antibodies from seeds. It gives a large biomass and is harvested several times a year.	It produces toxic compounds	[2, 15]
Potato (Solanum tuberosum)	One of the most used crops in laboratory experiments in terms of ease of handling and genetic modification. Easy to propagate with buds and tubers. Easy to store for long periods with little refrigeration needs.	It needs to be cooked, and this may lead to denature the antigen and reduce its effectiveness for immunization.	[12, 35]
Banana (Musa sapeintum)	It does not need cooking, and its proteins do not deteriorate even when cooking. Relatively cheap and grown in poor countries.	The tree needs 2–3 years to reach maturity. Transgenic trees need 12 months to bear fruit. The crop deteriorates quickly after harvesting, so the need for cold is an expensive process. It contains small amounts of protein, so it is difficult to produce large quantities of recombinant proteins.	[11, 36, 37]
Tomato (Solanum lycopersicum)	It is characterized by rapid growth, so it is grown in a wide range of environmental and geographical conditions. Containing vitamins C and A, which increase the immune response. The rapid spoilage of fruits can be avoided by freeze-drying techniques. The antigen is thermally stable. Tomato powder containing the antigen can be prepared in the form of powder after being freeze-dried, and it can also be made in the form of capsules. The possibility of mixing a group of antigens to be given as immunization doses from different diseases.	The fruits spoil quickly.	[1, 11]
Rice (Oryza sativa)	It is often used with baby food because it is non-allergic. Antigen with high expression of proteins.	Its growth is slow and it consumes large amounts of water.	[11, 36]
Lettuce (Lactuca sativa)	It grows fast and consumes quickly. It has a large biomass and can be grown in more than one season and in different geographical areas.	It cannot be transported over long distances after being genetically modified due to the possibility of its spoilage.	[5]
Carrot (Daucus carota)	Ease of cultivation with genetic modification and it consumed raw. Rich in antioxidants and vitamin A, which strengthens people with weak immunity in resisting the pathogen.	It needs good storage to ensure not denaturation of antigens.	[5, 12]
Algae (Chlamydomonas reinhardtii)	Having high rates of growth. The entire body of algae can be transformed genetically.	The production process is very expensive due to the use of bioreactors for fast growth of algae.	[34]
Pea (Pisum sativum)	It is characterized as a delicious food consumed by children and adults. The plant is seasonal, which helps to genetically engineer it and produce fruits in a short period of time. High protein content in seeds.	It needs a cooking process before being consumed as food, which reduces its immunogenicity.	[12]

Table 2.

Merits and demerits of plant species used as edible vaccines.

7. Conclusion

It is clear from the above that for many decades, traditional vaccines were the important factors in stimulating the human immune system to resist many diseases that affect him or to become immune to any vectors of diseases, whether viruses or bacteria. However, on the other side, the manufacture of these vaccines required a lot of effort, research, and the high cost of production, storage, and distribution around the world, as well as the occurrence of some side complications for these vaccines. This led many researchers in the past two decades to find alternative solutions by creating the idea of ​​edible vaccines and heading toward achieving this through the production of edible plant vaccines, which include edible plant parts from fruits, seeds, or plant products to be on hand for consumption by people as food. On the one hand, to stimulate human immunity to resist diseases or pathogens by genetically modifying these plants to contain antigens that fight diseases, in addition to making them easy to cultivate, store, and distribute worldwide as plant products. However, the limitations on this issue are that the concept is new and is not widely accepted at present in developing countries, and the opposition to transgenic plants by injecting them with special genes to make them edible vaccines. This needs to be conducted in many studies in the future.